If all else remains the same, the Xbox 360 redesign has set back the regular overtake rate by 3 weeks or so just this week alone. Probably once all is said and done, the Xbox 360 would have gained about a quarter or so assuming no other factor is involved.
Beyond that it gets messy. How do you quantify the effect of a hardware redesign? In the case of the PS3 slim there'd probably be very little change in demand as there was no significant advantage for people to get the new console over the old whereas with the Xbox 360 there were some very real reliability and noise concerns and even the lack of Wifi may effect sales somewhat as the Xbox 360 previously lacked this feature out of the box. Even a 5% change in base demand again ignoring everything else changes the longer term sales differential between the two systems but only in respect to timing.
Where it gets messier even still is the issue of price cuts and motion controls. It could mean nothing which results in the above factors likely netting another 3-6 months lead time for the Xbox 360 or it could mean a great deal and make the above irrelevant to the overall sales differential in the favour of the PS3 or Xbox 360.
Tease.
